Use + register for yc on Mac OS X as well

vim returns true for has('unix') on OS X and the has('xterm_clipboard')
returns false. But the + register works fine on OS X all the time, so add
an additional test to ensure that we enable the + register mapping all the
time on OS X.
Merge pull request #75 from ashcatch/master

Use + register for yc on Mac OS X as well